BACKGROUND OF THE INVENTION
1. Related Art
There are many known polymers which are comprised of an alternating sequence of rigid aromatic monomer units and other connecting units. These include many of the so-called engineering thermoplastics. Typical of these are the aromatic polycarbonates, polyarylethers, polyarylsulfones, and aromatic-aliphatic polyesters, such as poly(butylene teraphthalate) These polymers are either amorphous solids comprised of randomly coiled chains, or partially crystalline solids comprised of microcrystalline bundles of chains in which the local conformation (i.e., the conformation of a run of two or more repeat units along the chain) is in an extended conformation. Some of these engineering polymers can be oriented to give highly birefringent materials, but will not exhibit the large second-order nonlinear optical properties of the polymers of the present invention.
The polymer science literature also contains many descriptions of mainchain liquid crystal polymers in which the repeat units in the backbone are comprised of rigid mesogens or chromophores. Mesogens of neighboring chains tend to align parallel to one another in the solid state forming nematic or smectic phases. Some of these known mainchain liquid crystalline polymers also contain "flexible spacer units". The long axis of the rigid mesogenic repeat units of known polymers is aligned essentially parallel with the local axis of the polymer chain. This extended local conformation is illustrated by the formula: ##STR1## where the rigid mesogenic units (rectangles) are connected by flexible spacer units (connecting lines)
Aligning ground-state dipole moments in polarizable polymers imparts useful properties, such as second-order optical nonlinearity, and piezoelectric and pyroelectric properties.
There have been reports in the literature of sidechain and mainchain chromophoric polymers which can be poled in an electric field, or deposited by Langmuir-Blodgett techniques, to give films having second-order nonlinear optical properties. The mainchain chromophoric polymers reported to date, have the chromophoric dipole moments pointing in the same direction along the chain, in a head-to-tail arrangement. This configuration is not entirely satisfactory because of the difficulty in ordering all of the chains to lie in the same direction, which is a requirement for second-order nonlinear optical properties.
Electric field poling.
A useful process for the fabrication of second-order non-linear optical (NLO) polymer films is electric field poling. The dipole moments of the NLO polymers can be aligned by an electric field. The polymer is heated and poled above the glass transition temperature (Tg), then cooled below the Tg under the electric field. After the field is turned off, a net dipole moment can be locked into the film as long as the temperature remains well below the Tg. This imparts a noncentrosymmetry to the film which is necessary for important nonlinear optical properties.
Only recently have investigators paid close attention to dipole moments in rigid mainchain polymers. Polymers containing rigid chromophores in the main chain have been reported. Some of this effort was motivated by a search for superior nonlinear optical polymers. In these cases the mainchain repeat unit was in the head-to-tail configuration (isoregic). Hence, alignment of these known polymers by means of an electric field would tend to stretch out the backbone into a locally extended conformation. (see formula [1])
As an illustration, consider the polymer in the molten state near its glass transition temperature during electric-field poling. Under these conditions, the energy barriers to rotate and to align mainchain chromophores configured head-to-tail are so great that the degree of alignment has been shown to be less than that achieved in corona-poled sidechain chromophoric polymers under the same poling conditions. For the case of electric-field poling, to form useful nonlinear optical, piezoelectric or pyroelectric films, the accordion polymers of the present invention are superior to the known mainchain polymers because the degree of alignment of the chromophores will be higher in the accordion polymers.
The head-to-head pairs of chromophores with short flexible bridging groups of the present invention allow the polymer chain to "wrinkle" into an accordion conformation (above the glass transition temperature) under the influence of an electric field and to achieve a higher degree of alignment than can be attained with the head-to-tail (isoregic) chain. This accordion conformation is locked into the solid state by cooling the polymer in the electric field below its glass transition temperature. Once cooled down, the electric field is turned off and the folded conformation remains frozen in place for years, or until the polymer is reheated or stretched beyond its yield point.
Langmuir-Blodgett (LB) Deposition.
The Langmuir-Blodgett deposition process is well known in the art. Simply stated, an organic compound is floated on a liquid (usually water) surface, and a solid substrate is dipped through the interface depositing a single molecular layer per stroke on the substrate. The technology has matured to the point of having many commercial suppliers of computer-automated LB troughs.
Multilayer LB films can be formed in three different configurations, as illustrated by the formulas: ##STR2## Historically these are called "X"-, "Y"- and "Z"-type films, where X is made by depositing always on the down-stroke, Z is made by depositing always on the up-stroke, and Y is made by alternating up- and down-strokes. For the case in which the large dipole moment of the sidechain chromophore is normal to the polymer backbone (and the backbone is in the plane of the air-water interface), all-up or all-down films will be polarized, and the up-down films will not be polarized (dipoles in adjacent layers cancel out). The Y configuration is thermodynamically more stable (sometimes X and Z configurations spontaneously rearrange in the solid state to the Y configuration). Hence, one may interleave a sidechain chromophoric polymer with an optically inert spacer layer (having little dipole moment) and arrive at a stable, polarized film in the Y configuration.
One can purchase LB troughs equipped with two compartments such that multilayer films, alternating (AB).sub.n times, can be deposited automatically.
The bridging groups of the accordion polymers of the present invention can be selected to facilitate alignment on a LB trough For example, the degree of hydrophobicity (or hydrophilicity) of alternating bridging groups and the dipole of the main-chain repeat units can be tailored to achieve the desired orientation of the monolayer on the water surface. By deposition of these monolayers onto solid substrates, the polarized conformation of the accordion polymers can be maintained in multilayered films which will exhibit nonlinear optical properties, piezoelectric and pyroelectric properties.
Uses for NLO polymers.
Uses for the nonlinear optical properties include changing the direction, frequency, phase, polarity or amplitude of a laser beam transmitted through the polymer by incorporation of the polymer into a Pockels cell, an interferometer, an optical switch or an optical modulator. Piezoelectric materials convert pressure-volume energy or mechanical force into electrical energy (or vice verse) and can be used in pressure sensing (or sending) devices, such as accoustic membranes for speakers or sonar devices. Pyroelectric materials convert thermal energy into electrical energy (or vise versa) and are useful in devices used for sensing heat. As second-order nonlinear optical materials, these polymeric accordion polymers are especially useful for three-wave optical mixing. (See, Y. R. Shen, The Principals of Nonlinear Optics, John Wiley & Sons: New York, 1984)